1. Field of the Invention
This invention relates to a hydromechanical control for the displacement of a variable displacement hydraulic motor employed, for example, in a power drive unit.
2. Description of the Prior Art
Hydraulic motor driven actuating units or power drive units may be generally divided into two categories: those including a fixed displacement hydraulic motor and those including a variable displacement hydraulic motor. In the case of the fixed displacement motors, they are typically sized by a combination of stall torque, rated load and maximum speed required for their designed application. Because of the fixed displacement of the hydraulic motor, a considerable amount of hydraulic flow, and thus hydraulic power, is consumed at high speed operating conditions. Any difference between the hydraulic power drawn from the source and that delivered to the load will be dissipated in a servo valve that is typically utilized to control flow to and from the motor. This dissipation represents a power loss.
This power loss may be acceptable in some instances as a trade-off for the simplicity offered by a system employing a fixed displacement hydraulic motor. However, in many instances, the power loss is not acceptable. For example, the increasing secondary power requirements and decreasing heat sink capabilities of state-of-the art military and commercial aircraft may not permit and, in the future, will not permit, such power wastage.
Full, variable displacement hydraulic motors can realize a significant savings in hydraulic power for actuators over their fixed displacement counterparts, especially during high speed operation. However, this substantial improvement in efficiency is not without its cost. State-of-the-art methods for controlling variable displacement actuators typically involve an electronic controller with a plurality of differing control loops, with each loop requiring its own sensor. When one considers the redundancy typically required in aircraft control systems and avionics, an unsatisfactorily large cost for the multitude of sensors and controllers results.
Further, when employed where aiding loads or braking operations are required, the actuating systems may be pumping hydraulic fluid back into the hydraulic supply and distribution system; and some means must be added to accommodate the situation when the hydraulic motor acts as a pump in this fashion. For any of a variety of reasons, aircraft hydraulic system designers are reluctant to add any such means.
Thus, there is a real need for a simplified hydraulic actuator system that provides the energy savings of variable displacement systems but accomplishes the same without increasing electronic complexity, i.e., with the minimal sensing that is typically required only by fixed displacement hydraulic systems and wherein hydraulic fluid is not pumped back into the hydraulic system in the case of an aiding load or the like.